No, He (guide) set the price, do you deduct 20% if you don't reel in as many fish as expected?
Or do you pay the combine guy more if the yield is better, or less if it's worse?

It's mine (or your) money to spend how we see fit, but if a business sets their price, they should be happy getting paid that amount.
Do you tip Walmart an extra 15% if they have all the items on your shopping list?